`compile_as_cxx` should generate a path under` conf.build_dir`

If the current directory is different from `MRUBY_ROOT` and it has` conf.build_dir` and `conf.enable_cxx_exception` set, it was generating a pathname outside of` build_dir`.
As a result, in some cases files unrelated to mruby could be linked.

```console
% pwd
/tmp/mruby/1/2/3/4/5/6

% mruby_dir=/tmp/mruby/a/b/c/d/mruby

% cat my_config.rb
MRuby::Build.new("host", "build/to/custom/directory") do
  toolchain
  enable_cxx_exception
end

% rake MRUBY_CONFIG=my_config.rb -f $mruby_dir/Rakefile > logs
% grep CXX logs
CXX   a/b/c/d/mruby/src/error-cxx.cxx -> a/b/c/d/mruby/src/error-cxx.o
CXX   a/b/c/d/mruby/src/gc-cxx.cxx -> a/b/c/d/mruby/src/gc-cxx.o
CXX   a/b/c/d/mruby/src/vm-cxx.cxx -> a/b/c/d/mruby/src/vm-cxx.o
CXX   a/b/c/d/mruby/mrbgems/mruby-compiler/core/codegen-cxx.cxx -> a/b/c/d/mruby/mrbgems/mruby-compiler/core/codegen-cxx.o
CXX   a/b/c/d/mruby/mrbgems/mruby-compiler/core/y.tab-cxx.cxx -> a/b/c/d/mruby/mrbgems/mruby-compiler/core/y.tab-cxx.o
CXX   ../a/b/c/d/mruby/src/error-cxx.cxx -> ../a/b/c/d/mruby/src/error-cxx.o
CXX   ../a/b/c/d/mruby/src/gc-cxx.cxx -> ../a/b/c/d/mruby/src/gc-cxx.o
CXX   ../a/b/c/d/mruby/src/vm-cxx.cxx -> ../a/b/c/d/mruby/src/vm-cxx.o
```

0 files changed, 0 insertions, 0 deletions